Conventionally, in order to protect a motor from being burned, a protection element such as a bimetal or a PTC is built in a motor housing. When a motor abnormally generates heat, an electric circuit is interrupted by this protection element and the passing of electric current through the motor is stopped.
When the above-mentioned protection element is arranged near the motor, the size of the motor is enlarged, which results in increasing the size of an entire device. For this reason, a motor controller described in JP-H11-164472-A is not provided with the above-mentioned protection element but is constructed in such a way that a control section for driving and controlling the motor computes an estimated temperature of the motor from the magnitude of voltage applied to the motor, a period of time during which voltage is applied to the motor, and the last estimated temperature value. In the motor controller described in JP-H11-164472-A, when a computed estimated temperature becomes not less than a predetermined overheat protection temperature, the driving of the motor is stopped and the motor is held to be stopped until the estimated temperature reaches an overheat protection release temperature.
However, when the operating load of the motor is shifted from an assumption range in a design stage by a secular change or the like, the rotational speed of the motor is changed to increase the error of a computed estimated temperature. With this, it is likely that when the motor should not be stopped under normal circumstances, the motor is stopped by the function of protecting the motor from being burned or that when the motor should be stopped, the motor is not stopped and is burned.
Moreover, there is proposed a motor controller that sets a continuous operating time of a motor for the rotational speed of the motor and stops the motor when the operating time of the motor is over the continuous operating time. However, also in this case, it is likely that because of a difference in power supply voltage and a secular change, the function of protecting the motor from being burned detects an overheating state falsely or, conversely, does not detect an overheating state, which results in burning the motor. Moreover, in the above-mentioned motor controller, when the motor controller is mounted in a different system, the characteristics of the rotational speed of the motor and the continuous operating time need to be designed again, which results in increasing cost.